Two-dimensional Numerical Studies Research Articles

Two-dimensional numerical studies of particle motion and deposition in the channel of diesel particulate filters.

A numerical investigation on the soot laden flow of gas in a partial diesel particulate filter (PDPF) is presented based on solving the momentum equations for a continuous phase in the Euler frame and the motion equations for the dispersed phase in the Lagrangian frame. The interaction between the gas phase and the particles is considered as a one-way coupling for dilute particle concentration, while the interaction between particles and porous wall is implemented through user-definedsubroutines. To accurately track motion of nanoscale particles, the Brownian excitation and drag force as well as partial slip are taken into account in the particulate motion equation. Two methods are used to verify the gas flow model and reasonable agreements for both comparisons are observed. The effects of inlet velocity, wall permeability and particle size on the filtration efficiency and deposition distribution of the particles along with wall surface of inlet channel are quantitatively studied. The results show that (i) wall permeability plays the primary role in determining the filtration efficiency of PDPF, (ii) both upstream velocity and particle size have an effect on the initial deposition position of particles and (iii) filtration efficiency of PDPF is not markedly proportional to gas flow into inlet channels at a low wall permeability.

Numerical investigation of a Y-shaped thermoacoustic combustor with a Helmholtz resonator implemented and operated at off-design conditions

Self-excited thermoacousitc instability is undesirable in propulsion and gas turbine engines. To attenuate these instabilities, Helmholtz resonators shaped like a beer bottle are typically applied to dampen combustion-driven acoustic oscillations. The damping effect is maximized at resonance, since large volume of fluid in the cavity volume compresses and expands periodically, while a mass of the fluid in the resonator neck oscillates. Such periodic motions lead to thermos-viscous and vortex shedding losses. In this work, two-dimensional numerical studies on a Y-shaped thermoacoustic combustor with a Helmholtz resonator attached are conducted to shed lights on the damping performances of the resonators at off-resonance conditions. The Y-shaped system has a mother stem at the bottom. It splits into 2 bifurcating upper branches with different lengths corresponding to 2 non-harmonic longitudinal modes. Premixed propane and air are burnt in the bottom stem. The effects of the off-design conditions achieved by varying the resonator cavity volume on the limit cycle oscillations and the combustion products are examined in details. It is found that when the mass flow rate oscillation amplitude at the neck is increased by 1500%, the acoustic pressure amplitude is reduced by 20%. However, the dominant and secondary mode frequencies are almost unchanged.

Aeroelastic control of bridge using active control surfaces: Analytical and experiment study

An active control using a control surface system with winglets is described herein. Winglets help a long suspension bridge reach a stable state of flutter and buffeting. This system ensures the aerodynamic stability of a lightweight and economic stiffening girder without additional stiffening. Using a two-dimensional model, numerical and experimental studies were carried out to find a feedback control using winglets. In conclusion, a suitable control of the winglet motion can increase the flutter speed of a suspension bridge as the buffeting response moves in a contrasting manner.

Sensitivity analysis and lattice density optimization for sequential inherent strain method used in additive manufacturing process

Compensation of the thermal distortion that occurs during the fabrication process is an important issue in the field of metal additive manufacturing. Considering the problem in forming a lattice structure inside an object to reduce the thermal distortion, we developed a lattice volume fraction distribution optimization method. Assuming that the linear elastic problem is solved using the finite element method (FEM), an inherent strain method applying a layer-by-layer process utilizing the element activation during the FEM is formed as a recurrence relation, and the sensitivity of an objective function is derived based on the adjoint method. The unit lattice shape is a simple cube with a cube or a sphere-shaped air hole, and its distribution is optimized by considering the minimum thickness of the wall surrounding it as a design variable. The effective stiffness tensor of the lattice is derived using a homogenization method. The functions of the effective properties with respect to the design variables are approximated through polynomial functions. The optimization problem is formulated as an unconstrained minimization problem. The design variables are optimized using the method of moving asymptotes. Herein, the validity of the proposed method is discussed based on quasi two-dimensional and three-dimensional numerical studies including a re-analysis through full-scale thermo-mechanical analysis.

Development of an excavatability test for backfill materials: numerical and experimental studies

This paper describes research done on a new testing device including its laboratory test procedure; namely, a laboratory punching test for cementitious backfill materials that evaluates their excavatability with a pick. Three-dimensional and two-dimensional numerical studies using a finite element method were carried out according to calibrated parameters of the material obtained by triaxial tests. This made it possible to study the behavior of the backfill material under action induced by a tool (a pick) as well as to optimize the geometry of the punching test specimen. This paper provides detailed information and results for the experimental study of this laboratory test. Attention was particularly focused on energy relating to displacement as well as types of ruptures. The behavior of these types of materials was found to be greatly influenced by gravel in the material. Repeatability of the test was also studied. Finally, a method for this punching test of backfill materials is also provided.

Entropy production in a fluid-solid system far from thermodynamic equilibrium.

The terminal orientation of a rigid body in a moving fluid is an example of a dissipative system, out of thermodynamic equilibrium and therefore a perfect testing ground for the validity of the maximum entropy production principle (MaxEP). Thus far, dynamical equations alone have been employed in studying the equilibrium states in fluid-solid interactions, but these are far too complex and become analytically intractable when inertial effects come into play. At that stage, our only recourse is to rely on numerical techniques which can be computationally expensive. In our past work, we have shown that the MaxEP is a reliable tool to help predict orientational equilibrium states of highly symmetric bodies such as cylinders, spheroids and toroidal bodies. The MaxEP correctly helps choose the stable equilibrium in these cases when the system is slightly out of thermodynamic equilibrium. In the current paper, we expand our analysis to examine i) bodies with fewer symmetries than previously reported, for instance, a half-ellipse and ii) when the system is far from thermodynamic equilibrium. Using two-dimensional numerical studies at Reynolds numbers ranging between 0 and 14, we examine the validity of the MaxEP. Our analysis of flow past a half-ellipse shows that overall the MaxEP is a good predictor of the equilibrium states but, in the special case of the half-ellipse with aspect ratio much greater than unity, the MaxEP is replaced by the Min-MaxEP, at higher Reynolds numbers when inertial effects come into play. Experiments in sedimentation tanks and with hinged bodies in a flow tank confirm these calculations.

Two-dimensional modeling of a plant-scale fixed-bed reactor for hydrogen production from methanol steam reforming

Methanol as hydrogen carrier can be reformed with steam over CuO/ZnO/Al2O3 catalysts. Two-dimensional pseudo-homogenous modeling and numerical simulation studies have been carried out for methanol steam reforming reactor integrating with global kinetics of methanol decomposition and methanol steam reforming. Hydrogen output and thermal behavior were investigated with respect to parameters such as mass flow rate and inlet mass fraction of methanol, external thermal oil inlet temperature and tube diameter. Good agreement was achieved between plant data and simulated data. The results show that temperature distribution changed more dramatically than concentration distribution along axial. Hydrogen production had a positive relationship with methanol mass flow rate, thermal oil inlet temperature. Thermal behavior along radial can be greatly affected by all discussed operating conditions. Large tube diameter was adverse to heat transfer leading to high temperature difference. These simulations can guide for manufacture operation and design of plant-scale reactor.

Effect of Thermophysical Properties of Solid Walls on Turbulent Modes of Complex Heat Transfer in an Enclosure

Two-dimensional numerical studies were performed for investigation of the effect of thermophysical properties of solid walls on turbulent <em>convective </em>– <em>radiative </em>heat transfer in an air filled square cavity. The <em>main attention was paid to the influence of </em>thermal conductivity ratio1 ≤ κ1, 2 ≤ 1000 and an emissivity of internal surfaces of the solid walls 0 £ < 1 on velocity and temperature fields. Numerical results were obtained by means of the finite difference method. The effect of the governing parameters on the average Nusselt number has been defined. The obtained results provide better technical support for development and research of energy-efficient building materials

Three- and two-dimensional simulations of counter-propagating shear experiments at high energy densities at the National Ignition Facility

Three- and two-dimensional numerical studies have been carried out to simulate recent counter-propagating shear flow experiments on the National Ignition Facility. A multi-physics three-dimensional, time-dependent radiation hydrodynamics simulation code is used. Using a Reynolds Averaging Navier-Stokes model, we show that the evolution of the mixing layer width obtained from the simulations agrees well with that measured from the experiments. A sensitivity study is conducted to illustrate a 3D geometrical effect that could confuse the measurement at late times, if the energy drives from the two ends of the shock tube are asymmetric. Implications for future experiments are discussed.

Conceptual design and numerical simulations of a vertical axis water turbine used for underwater mooring platforms

Energy is a direct restriction to the working life of an underwater mooring platform (UMP). In this paper, a vertical axis water turbine (VAWT) is designed to supply energy for UMPs. The VAWT has several controlled blades, which can be opened or closed by inside plunger pumps. Two-dimensional transient numerical studies are presented to determine the operating performance and power output of the turbine under low ocean current velocity. A standard k-e turbulence model is used to perform the transient simulations. The influence of structural parameters, including foil section profile, foil chord length and rotor diameter, on the turbine performance are investigated over a range of tipspeed-ratios ( TSRs ). It was found that turbine with three unit length NACA0015 foils generated a maximum averaged coefficient of power, 0.1, at TSR = 2.

A SPACE–TIME CE/SE METHOD FOR SOLVING HYPERBOLIC HEAT CONDUCTION MODEL

This paper is concerned with the numerical approximation of one and two-dimensional models describing heat conduction in solids at low temperature. The system contains a conservation equation for the energy density and balance equations for the heat fluxes along each characteristic direction. A conservation element and solution element method (CE/E) is proposed for solving the given model. The method has already shown its efficiency and accuracy for solving a wide range of engineering problems. This technique is considerably different in both logic and design from the well-established finite volume schemes. The scheme treats space and time in a unified manner where conserved variables and their gradients are considered as independent unknowns. To illustrate the performance of the CE/SE method, several one and two-dimensional numerical case studies are carried out. The results of kinetic flux vector splitting (KFVS) scheme and central scheme are also presented for the comparison and further validation of our numerical results. The computations of this paper demonstrates that CE/SE method is effective in handling such problems.

Special issue on patient specific modelling (PSM)

This special issue of the journal includes nine papers on patient specific modelling (PSM)—five related to cardiovascular modelling, three on the simulation of musculoskeletal systems and one on brain biomechanics. Typically in PSM, anatomy and other data specific to an individual are fed into a computer programme, which calculates and delivers information relevant to the specific individual to enable provision of patient-specific diagnoses, treatment or prognosis. Examples include information on the following: blood flow and related quantities such as wall shear stress obtained using computational fluid dynamics, strain and stress in soft tissues or bone using computational solid mechanics or electrical activity in the heart using electrical models. The medium to long-term aim in this field is the provision of information that is clinically useful, to predict the risk of clinical events such as aneurysm rupture or stress fracture, to categorise patients for surgery, to aid surgical planning and in the prediction of post-surgery events. The area of PSM has its origins in two-dimensional numerical modelling studies using idealised geometries [1–4]. Advances in high-resolution three-dimensional (3D) medical imaging have allowed acquisition of geometries suitable for PSM. The first reports that can be classified as being subject specific in cardiovascular modelling were of blood flow in arteries [5–8]. For musculoskeletal modelling, automated procedures to generate 3D finite element meshes from scans provided a major boost to PSM [9, 10]. In this special issue three cardiovascular modelling papers describe applications in abdominal aortic aneurysms: Doyle et al. [11] investigated the effect of different material properties on the model outputs, Hardman et al. [12] investigated inlet flow data and Morris et al. [13] investigated the effect of different stent configurations. Different treatment options to improve hemodynamics in patients with total cavopulmonary connections are described by Mirabelle et al. [14], and Smith et al. [15] described a framework for simulation of cardiac perfusion. Pankaj [16] reviewed the challenges associated with assigning material properties and boundary conditions in the PSM of bone and bone implant systems, whereas a review on the constitutive behaviour of trabecular bone is provided by Carretta et al. [17]. Taylor et al.[18, 19] considered the PSM of the hip and knee, pre-implantation and post-implantation. A fuzzy mesh-free framework for the PSM of the brain is described by Zhang et al. [19]. The current state of PSM is reflected by the papers in this special issue in that there remains an emphasis on the development of methodology and optimisation of the processing chain, whereas a few observational studies have been published comparing data from different patient groups. Clinical trials whose aim is to establish the clinical utility of PSM have yet to be published, whereas several studies are in progress at the time of writing. General reviews of PSM are provided by several authors [20, 21].

Tornadogenesis in a Simulated Mesovortex within a Mesoscale Convective System

Abstract The Advanced Regional Prediction System (ARPS) is used to simulate a tornadic mesovortex with the aim of understanding the associated tornadogenesis processes. The mesovortex was one of two tornadic mesovortices spawned by a mesoscale convective system (MCS) that traversed southwestern and central Oklahoma on 8–9 May 2007. The simulation used 100-m horizontal grid spacing, and is nested within two outer grids with 400-m and 2-km grid spacing, respectively. Both outer grids assimilate radar, upper-air, and surface observations via 5-min three-dimensional variational data assimilation (3DVAR) cycles. The 100-m grid is initialized from a 40-min forecast on the 400-m grid. Results from the 100-m simulation provide a detailed picture of the development of a mesovortex that produces a submesovortex-scale tornado-like vortex (TLV). Closer examination of the genesis of the TLV suggests that a strong low-level updraft is critical in converging and amplifying vertical vorticity associated with the mesovortex. Vertical cross sections and backward trajectory analyses from this low-level updraft reveal that the updraft is the upward branch of a strong rotor that forms just northwest of the simulated TLV. The horizontal vorticity in this rotor originates in the near-surface inflow and is caused by surface friction. An additional simulation with surface friction turned off does not produce a rotor, strong low-level updraft, or TLV. Comparison with previous two-dimensional numerical studies of rotors in the lee of mountains shows striking similarities to the rotor formation presented herein. The findings of this study are summarized in a four-stage conceptual model for tornadogenesis in this case that describes the evolution of the event from mesovortexgenesis through rotor development and finally TLV genesis and intensification.

On the [formula omitted]-adaptive solution of complete electrode model forward problems of electrical impedance tomography

Electrical impedance tomography is a noninvasive imaging technique based on measurements of currents and voltages on the boundary of the object of interest. The most accurate forward model for impedance tomography is the complete electrode model that takes into account the electrode shapes and the contact impedances at the corresponding interfaces; many practical reconstruction algorithms of electrical impedance tomography require repetitive computation of accurate forward solutions for the complete model. In this work, we introduce an hp-adaptive finite element method for solving the complete electrode forward problem and demonstrate its functionality via two-dimensional numerical studies.

Three-dimensional volume of fluid simulations on bubble formation and dynamics in bubble columns

It is a key step to understand the bubbling behavior in order to quantitatively describe the flow and transfer behavior in bubble columns. Compared with two-dimensional simulation, three-dimensional simulation is able produce relatively realistic results of bubble behavior. Based on two-dimensional numerical studies on single bubbling behavior, three-dimensional numerical simulations on bubble formation, growth, detachment, rise motion with one-, two- and four-orifice and interaction of two parallel bubbles in bubble columns were investigated using the volume of fluid (VOF) model on the software platform of Fluent 6.3, and the results were validated with the experimental observations. Effects of fluid physical properties, operation conditions and orifice size on bubble behavior were numerically analyzed besides the interactions of parallel bubbles in bubble columns and the variation rules on bubble formation and rising behavior were obtained.

The three-dimensional wake of a cylinder undergoing a combination of translational and rotational oscillation in a quiescent fluid

Previous two-dimensional numerical studies have shown that a circular cylinder undergoing both oscillatory rotational and translational motions can generate thrust so that it will actually self-propel through a stationary fluid. Although a cylinder undergoing a single oscillation has been thoroughly studied, the combination of the two oscillations has not received much attention until now. The current research reported here extends the numerical study of Blackburn et al. [Phys. Fluids 11, L4 (1999)] both experimentally and numerically, recording detailed vorticity fields in the wake and using these to elucidate the underlying physics, examining the three-dimensional wake development experimentally, and determining the three-dimensional stability of the wake through Floquet stability analysis. Experiments conducted in the laboratory are presented for a given parameter range, confirming the early results from Blackburn et al. [Phys. Fluids 11, L4 (1999)]. In particular, we confirm the thrust generation ability of a circular cylinder undergoing combined oscillatory motions. Importantly, we also find that the wake undergoes three-dimensional transition at low Reynolds numbers (Re≃100) to an instability mode with a wavelength of about two cylinder diameters. The stability analysis indicates that the base flow is also unstable to another mode at slightly higher Reynolds numbers, broadly analogous to the three-dimensional wake transition mode for a circular cylinder, despite the distinct differences in wake/mode topology. The stability of these flows was confirmed by experimental measurements.

Interaction of turbulent natural convection and surface thermal radiation in inclined square enclosures

Two-dimensional numerical studies of flow and temperature fields for turbulent natural convection and surface radiation in inclined differentially heated enclosures are performed. Investigations are carried out over a wide range of Rayleigh numbers from 108 to 1012, with the angle of inclination varying between 0° and 90°. Turbulence is modeled with a novel variant of the k–e closure model. The predicted results are validated against experimental and numerical results reported in literature. The effect of the inclination of the enclosure on pure turbulent natural convection and the latter’s interaction with surface radiation are brought out. Profiles of turbulent kinetic energy and effective viscosity are studied to observe the net effect on the intensity of turbulence caused by the interaction of natural convection and surface radiation. The variations of local Nusselt number and average Nusselt number are presented for various inclination angles. Marked change in the convective Nusselt number is found with the orientation of enclosure. Also analyzed is the influence of change in emissivity on the flow and heat transfer. A correlation relevant to practical applications in the form of average Nusselt number, as a function of Rayleigh number, Ra, radiation convection parameter, N RC and inclination angle of the enclosure, φ is proposed.

A super beta bipolar transistor using SiGe-base surface accumulation layer transistor(SALTran) concept: a simulation study

Current gain is an important design parameter of bipolar transistors. While a SiGe base is commonly used to increase the current gain, the recently reported surface accumulation layer transistor (SALTran) concept has been shown to give a similar current gain enhancement. Using two-dimensional numerical simulation studies, we show for the first time that a combination of the SiGe base and the SALTran concept can be used to realize super beta bipolar transistors with peak current gains more than 12000.

Enhanced breakdown voltage and reduced self-heating effects in thin-film lateral bipolar transistors: Design and analysis using 2-D simulation

In this work, we present our two-dimensional numerical simulation studies and analysis of the enhanced breakdown and self-heating characteristics of a new collector-tub three-zone step doped thin-film lateral bipolar transistor (CT-SLBT) on silicon-on-insulator (SOI), which shows enhanced breakdown voltage as high as 80% when compared with that of the conventional uniformly doped lateral bipolar transistor (LBT) on SOI. The design issues and the reasons for the improved performance are discussed in detail.

Effects of rotation on natural convection cooling from three rows of heat sources in a rectangular cavity

Two-dimensional unsteady numerical studies are made on an air-filled enclosure rotated about its horizontal axis with an array of three rows of heat sources on one of the walls, revealing three physically realizable phenomena, namely, uni-periodic oscillation, multi-periodic oscillation and chaotic oscillation. The evolutionary process of flow field and natural convection characteristics from stationary to rotating situation is studied. Rotation results in an imbalance between clockwise and counter-clockwise circulations, increases heat transfer in the worst scenario, reduces the oscillation of Nusselt number, and improves or reduces mean performance in each cycle. The optimal distribution of heaters in rotating fluid is close to the results in the stationary situation if they have same dominated circulation direction.